How Do Elliptical Galaxies Form?

1. How Do Elliptical Galaxies Form? Dunbar Vocational Career Academy. Adarra Benjamin, Jimmie Butler, Hakim Corner, DeMarco Ewings University of Chicago. Our Project Looking for Merged/Merging Galaxies Background Search • We were accepted in the NASA Experience project for the Chicago Public Schools, and learned about astronomy at the University of Chicago. The program included lectures and laboratories about stars, galaxies, and the Universe, as well as the tools astronomers use to study them. We learned about optics, and how astronomers use diffraction gratings to produce spectra of objects in the sky to classify and study them. We learned that astronomers make use of large data bases of pictures and spectra of stars, galaxies and other objects to locate interesting objects to study. One such database is the Sloan Digital Sky Survey (SDSS). We learned how to find objects of interest in the Sloan data base using Structured Query Language (SQL). Using SQL, we can download information about stars’ and galaxies’ coordinates, color magnitudes, redshift, etc. • We became interested in how elliptical galaxies formed. We were curious how a spiral galaxy “loses” its spiral arm structure to become a more homogeneous distribution of stars seen in a typical elliptical galaxy. One theory some astronomers have is that spiral galaxies merge, and as they do, gravitational interactions degrade the arm structure. This lead us to want to find galaxies in the process of merging. A histogram is a graph that shows how many occurrences of an object there are in a bin. So we made a histogram of galaxies that are in certain g-magnitudes bins like 19.5-20, 20-20.5, etc. The bin represents the distance from one point to another point and the count is how many galaxies are contained in the region. A spiral galaxy is blue because it has a lot of new stars and new stars are blue. A elliptical galaxy is red because it has a lot of old stars: old stars are red. Massive stars will begin to die out, a process that causes them to get redder; so blue stars turn to red stars in a matter of millions of years. The purpose of the histogram was to find the “Green Valley.” A “Green Valley” is a point where neither the blue or the red are dominant in the spectra. If you look at the histograms below, you can see that the minimum occurs at u-r=2.2. This minimum helped find the “Green Valley.” We took the data that we got from Sloan data base and we put it into Microsoft Excel. We did that to get the cooridnates, right ascension and declination, and we put it in to the SDSS DR7 Image list tool to get the images of the galaxies from Sloan… after doing that we look through the images to figure out if we could find any images of galaxies that might be close to merging. For example, we download slices of data based on magnitude of the galaxies and their position in the green valley. We were looking for images with two galaxies that were close enough to merge. We determined they were close enough by looking at the redshift of the two galaxies and seeing if they were the same or similar to each other. After that we figured if it was a merger or if it was just two galaxies next to each other. Data Analysis We made a color separation graph between spiral galaxies and elliptical galaxies. We plotted the magnitude in a blue filter minus the magnitude in a red filter against the magnitude in a green filter. The graph showed galaxies with blue spectra toward the left side and galaxies with red spectra on the right. We examined pictures and spectra for 120 galaxies at random in the blue region and another 120 in the red area. The galaxies in the blue region were all spirals, based on their individual spectra and images. Similarly, the galaxies we looked at from the red area all had spectra that showed them to be elliptical galaxies. About 60 galaxies were selected from the area between the blue and red regions, and we found a mix of spiral and elliptical galaxies. This area is where we suspected we might find spiral and elliptical galaxies merging, because their spectra were neither strongly blue nor strongly red. We conducted further analysis to pinpoint this region where we expected to find merging galaxies. Results Project Introduction • We became interested in galaxies. There are two types of galaxies: spiral and elliptical. Spiral galaxies are relatively young galaxies and are composed of young, hot stars. These young hot stars burn hydrogen, and shine in the blue portion of the color spectrum. Spiral galaxies also appear to have “arms” of stars that swirl around a center, looking like glowing spirals. Elliptical galaxies are much older. Its stars have pretty much burned their supply of hydrogen in the core, and have cooled. They burn helium and other elements, and generally have spectra that are bright in the red portion of the color spectrum. Spiral Galaxy Images Elliptical Galaxy Images Further Research During our search, we found several interesting objects such as the one pictured here, that are worthy of further study, perhsaps as a Science Fair Project. Also, we used fairly simple SQL queries. It is possible that more merging galaxies could be found with a sophisticated query that would match galaxies with similar redshifts in the same frame. Typical Spiral Spectrum Typical Elliptical Spectrum Acknowledgements All photos and spectra were obtained from the Sloan Digital Sky Survey database. We wish to thank Dr. Don York, Alan Zablock, Mark Casey and Russ Revzan for their suggestions and help on this project.. Galaxies in the left section are blue and galaxies in the right. The minimum near the center is called the “Green Valley.”